When did Agatha Christie become best-seller writer ?

There are a lot of informations about the total amount of books sold by Agathe Chrstie, but it's very difficult to get an idea of the progression. I know she sold 8000 issues of "Morder of Roger Akroyd", but what was the progression after. When did she become a best seller writer, with which book, how did her sellings increase in England and in the world. I'm looking for informations about the progression of her success and her fame. Thank You! Jean-Marie Bretagne (Paris)